I've been hearing that the G36/SL8 series is about as clean as can be, from firing. No fouling, etc.
Having said that, I haven't "borrowed" my father's SL8 and taken it out yet, so I can't speak from experience.
However...it seems prudent in the very least to take it down, and clean it from top to bottom, Just In Case(tm). At the very least, you'll find out how much fouling is already present, and learn how to lube it properly, as well as learn how to take it down from memory for later..
I've picked up exactily one rifle from a dealer that I've fired without cleaning it beforehand myself. I was not happy with the results at the range (jams, etc). A cleaning solved all that. Every thing else, I've cleaned before loading up the mags and heading to the range.
Besides, you never know in what condition they left it in. Best to make sure before you take it out.